SAN FRANCISCO--For everyone who has wondered where to live to avoid getting rattled by an earthquake, a new map provides some hints. Today, earth scientists released the first global map that shows the odds of severe ground shaking during earthquakes for the next 50 years. The map will help engineers design safer buildings in regions that face high hazards, especially in countries where researchers had not previously defined seismic risks.
Many geological atlases chart the world's major faults and pinpoint where big quakes have struck in the last century or longer. However, such maps don't reveal the strength of ground shaking, which varies greatly depending on the nature of the earthquake and the landscape around it. For instance, a giant quake off the coast or deep underground may produce only mild shaking in the closest city, while a small quake can wreak havoc if its fault breaks the surface where people live. Other factors include how frequently a section of the fault breaks and whether the ground consists of solid rock or loose soils, which can amplify shaking.
The new map, unveiled here today at a meeting of the American Geophysical Union, takes all such factors into account to quantify the chances that a particular region will feel shaking motions ranging from negligible to severe. More than 500 seismologists and geologists worked for 6 years, using satellite measurements, field surveys, historical records of earthquakes, and geologic evidence of ancient ground motions to document the hazards for all land areas on Earth, says the project's director, Domenico Giardini of the Swiss Seismological Service in Zürich. Engineers and seismologists in more than half of the world's countries had few such maps or had waited until major quakes struck to calculate the risks of future shaking, Giardini notes. "[The new hazard map] is the first step to understand and mitigate seismic risk," he says.
The worst hazards--defined as a 10% chance of ground accelerations exceeding half the force of gravity within the next 50 years--exist in southern California, southeastern Hawaii, Iceland, Taiwan, Turkey, and the seismic collision zone between India and China, says seismologist Kaye Shedlock of the U.S. Geological Survey in Denver, Colorado. About 15% of the world's land areas--and more than half of the world's cities with at least 3 million people--face what Shedlock calls "high to very high risk" of severe shaking in the next few decades. However, the map's resolution remains coarse: just one hazard value for every 100-square-kilometer parcel of land. "Our goal is [for that] to be replaced within a few years" by more detailed maps for the high-risk regions, Shedlock says.